A United Airlines passenger gets boarding documents from a kiosk at San Francisco International Airport.

United Airlines doesn't offer a nonstop flight between Baton Rouge and Istanbul. It's a nearly 23-hour trip involving two other airlines. In an effort to dull some of the pain, and perhaps a sprint around an unfamiliar airport, United is now giving passengers access to their boarding passes on connecting legs provided by other airlines.

Travelers using United's app can download all of their boarding passes on itineraries that include both United and one of its 19 partner carriers, such as Lufthansa or Turkish Airlines, the carrier said Tuesday. It added that it's the first U.S. carrier to offer the feature.

The move makes the humble boarding pass the latest battleground for airlines.

There are a few catches, however. United travelers won't be able to use the feature if the first leg of their trip isn't on a United flight, a spokeswoman told CNBC. That means it can't be used on some U.S.-bound itineraries.

Also, travelers who separately book a ticket in United's basic economy, a new class of service that prohibits passengers' access to overhead bins and seat assignments in exchange for a lower fare, are not able to combine tickets with other airlines, so they can't access the feature, the spokeswoman added.